SPARK-19726: change the value to true in the caller side

Author: shuangshuangwang

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -61,7 +61,7 @@ object JDBCRDD extends Logging {
       try {
         val rs = statement.executeQuery()
         try {
-          JdbcUtils.getSchema(rs, dialect)
+          JdbcUtils.getSchema(rs, dialect, true)
         } finally {
           rs.close()
         }

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JdbcUtils.scala

@@ -273,7 +273,7 @@ object JdbcUtils extends Logging {
   def getSchema(
       resultSet: ResultSet,
       dialect: JdbcDialect,
-      alwaysNullable: Boolean = true): StructType = {
+      alwaysNullable: Boolean = false): StructType = {
     val rsmd = resultSet.getMetaData
     val ncols = rsmd.getColumnCount
     val fields = new Array[StructField](ncols)